The present invention relates to a gas-cooled electrical machine having an axial fan.
As machine utilization increases, the requirements for the cooling of rotating electrical machines become ever greater. This applies in particular to gas-cooled turbogenerators and, to an even more significant extent, to turbogenerators having an indirectly cooled stator winding. In machines of this type, all the heat losses produced in the stator winding have to be dissipated through the winding insulation into the cooled laminated stator core.
There has thus been no lack of proposals to improve the cooling, in which case one fundamental refinement of the cooling circuit has been found to be very effective, namely the change from conventional pressure cooling to suction cooling. In general, suction cooling offers the advantage over pressure cooling that the cold air which is supplied from the coolers flows directly to the cooling channels in the generator stator and generator rotor. The temperature increase produced by the machine fan can thus be completely eliminated.
With the aim of constructing turbogenerators with higher specific ratings, suction cooling is limited, however, not least by the operating conditions. Limit-rating machines are in general likewise subject to the permanent aim of increasing the unit rating, as a consequence of which the requirements for the cooling system are growing continuously. The core and driving force of the generally closed suction cooling circuit in the generator is in this case a fan which is seated on a rotor shaft and feeds the required amount of cooling flow through the branches of the cooling path system. In the case of air-cooled generators with suction cooling, a relatively large cooling volume flow is generally required with a low to medium pressure difference, in order to overcome the cooling circuit resistance. Thus, in principle, fans of an axial type are preferred. Such a generator with suction cooling has been disclosed, for example, in European Patent Application 840 426.
In accordance with fundamental flow rules, axial fans operate efficiently only provided the flow design of the inlet and outlet parts allows axial streamlined guidance in the area of the blades. Furthermore, the implementation of a downstream guidance mechanism is required for controlled pressure recovery from the high kinetic energy at the rotor outlet, in order to increase the efficiency of the fan stage or to reduce the required drive power for the fan for a given volume flow and pressure increase, since this drive power significantly influences the balance of losses in the generator.
Conventional efficiency improvement measures relating to the design of the inlet and outlet areas of axial fans generally occupy an enormous amount of space axially as is described, by way of example, in EP 682 399. However, in the case of generators having a high unit rating, the fundamental aim is to maximize the proportion of the electrically usable active partial length on the impeller length between the bearings, with the impeller length being limited for mechanical reasons and reasons related to shaft dynamics. A known combination of an impeller of the axial fan and a guide wheel with blades has the disadvantage, in addition to a disproportionately large axial space requirement, of a high noise level, which is a result of the interaction of the guide wheel blade system and the wake depressions generated upstream by the rotating impeller blades.
Starting from the prior art, the invention is therefore based on the object of providing an efficiently operating axial fan stage with a downstream guidance mechanism in an axial physical space which is as small as possible, in a turbogenerator having a suction cooling circuit, in a simple and economic manner.
This object is achieved according to the invention by the features of the first claim and, in particular, by a downstream guidance mechanism, arranged downstream of an axial fan, as a deflection diffusor for significantly raising the pressure and deflecting the cooling gas. With a given arrangement and subsequent installation of such a deflection diffusor, the pressure increase results in a considerable increase in the cooling volume flow while, in contrast, for a given cooling air flow, the deflection diffusor allows the power consumption of the fan to be reduced, and thus allows the efficiency of the overall electrical machine to be increased.
A particularly advantageous refinement of the invention is distinguished by the fact that the deflection diffusor comprises a diffusor funnel and a diffusor wall, in which case the diffusor funnel is arranged on a partition wall, which separates the inlet area of the axial fan from its outlet area, in the plane of the axial fan. The diffusor wall is arranged downstream of the diffusor funnel.
Furthermore, it is particularly advantageous for the diffusor wall to be designed in an annular shape and to be arranged parallel to an outer casing of a generator.
A further preferred refinement is distinguished by the fact that the diffusor wall is designed in the shape of a conical envelope, and its outer radius is thus at a shorter distance from the outer casing than its inner radius.
Furthermore, it is particularly advantageous for a number of guide vanes to be arranged between the diffusor wall and the outer casing, in order to increase the pressure further.
Further advantageous refinements of the invention can be found in the dependent claims.